Various types of loud speaker driver units generate sound in response to an electrical signal to the speaker have been developed. Known loud speakers include a motor that acts as a transducer of electrical energy to mechanical energy. A radiating diaphragm then transduces the mechanical energy into acoustical energy. With reference to FIG. 1, a conventional audio speaker 1 includes a cone-shaped diaphragm 2 that is operably interconnected with a chassis 3 via a suspension system such as a flexible surround 4. The speaker 1 includes an inner suspension consisting of a flexible member commonly referred to as a “spider” 5. The rear or inner suspension may also be referred to as a “damper”. A voicecoil 6 is formed of wire wound around a voicecoil former 7. The former 7 is also commonly referred to as a “bobbin”. Terminals 9 are secured to the frame 3, and are electrically interconnected to the voicecoil 6 via flexible leads 8. The flexible leads 8 are also commonly referred to as “tinsel leads”. A magnetic assembly 10 includes a ring magnet 11 and center pole 12 that are secured to a back plate 13.
Prior art low frequency loud speakers, or woofers, such as illustrated in FIG. 1, are typically quite deep, such that the speakers may take up a substantial amount of space. The depth is the result of the stack up of dimensions of cone depth, cone apex to magnet top plate clearance, clearance for attachment and operation of the rear suspension, voicecoil length, clearance for rear of the voicecoil to the magnet back plate, and back plate thickness. The clearance dimensions and voicecoil length are largely determined by the maximum rearward excursion required for a particular design. The diaphragm of a loud speaker converts the force generated by the motor to acoustical radiation. All else being equal, the larger the radiating area of the diaphragm, the greater the acoustical output. In addition, all else being equal, the greater the axial excursion of the diaphragm, the greater the acoustical output. The requirements for area and excursion for a given output increase quickly as frequency decreases. These requirements have led to large woofers with long excursion capability.
In general, there are three primary loads on the diaphragm against which the voicecoil force is applied. First, acceleration of the diaphragm and air masses, and part of the suspension. Second, a load results from the compression or rarefaction of the air volume of the system enclosure. Third, compression and extension of the spring stiffness of the outer suspension also generates a load on the diaphragm. In general, the load of the radiated acoustic power for a direct radiating woofer is negligibly small.
These loads cause the diaphragm to flex, thus causing a loss of acoustic radiation, and potentially causing structural failure. Acceleration and air compression loads are distributed over the entire area of the diaphragm rather than being concentrated in a small area. On the other hand, drive force from the voicecoil and load from the perimeter suspension mass and spring stiffness are applied at inner and outer rings of high force concentration. Accordingly, these rings must be designed to prevent structural failure.
In prior art cone or thin parallel plate flat diaphragm type speakers, reinforcing coupling members may be required to spread the force from the motor. Such reinforcing is generally not required at the outer perimeter of the diaphragm because the attached mass is low and the length of attachment is relatively great. Nevertheless, delamination of cone paper or separation of skin from the core of the diaphragm may occur in such speakers.
In many applications, a system enclosure may easily accommodate a woofer having a relatively large depth. However, for other applications, a woofer having a relatively large depth may take up an unacceptably large amount of space. Examples of such applications include car doors, in-wall, and under-seat woofers. Thus, a speaker having relatively poor low frequency capability may ultimately be used in such applications due to the space constraints.
Several approaches have been tried in an attempt to provide a low frequency speaker having a shallow overall dimension. One approach involves reducing the depth of the diaphragm cone. However, this approach results in increased cone flexure, which can lead to failure and loss of effective volume displacement. Also, a relatively flat cone also has less resistance to axial tilt because the surround and spider are moved closer together, reducing the lever arm that resists tilt. Excess axial tilt may cause the voicecoil to contact the magnet poles, causing distorted sound and reduced reliability. Another approach involves reducing excursion to allow reduction of clearance in the axial direction, thereby providing low frequency speaker that is relatively shallow. However, this approach results in a direct sacrifice of performance for the reduced depth due to the reduced excursion. Yet another approach that has been attempted utilizes an inverted motor that places the magnetic assembly and voicecoil inside the cone, thus utilizing previously unused space. However, a substantial extension of the voicecoil former is necessary to provide clearance between the front cone surface and the front surface of the top plate of the magnet assembly. This results in the magnet assembly being positioned forward in the cone, such that it protrudes beyond it, thus increasing depth.